Encapsulation method for electronic modules

ABSTRACT

An encapsulated electronic device includes a magnetically permeable core structure which is exposed within and coplanar with a flat top surface of the device. A bottom surface of the core may be exposed within the bottom surface of the device. The bottom core surface may be recessed beneath, coplanar with, or protruding from the bottom surface of the device. Alternatively the bottom surface may be encapsulated within the device. A method for manufacturing the exposed core package includes positioning a first component relative to a second component before encapsulating the device. An improved planar magnetic core structure includes internal bevels having a radius greater than or equal to 15% and preferably 25%, 35%, or as much as 50% of the core thickness to reduce concentration of the magnetic field around the internal corners.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims priority to,U.S. application Ser. No. 12/493,773, filed on Jun. 29, 2009. Thisapplication is related to U.S. application Ser. No. 12/493,759, filed onJun. 29, 2009, titled “Encapsulation Method and Apparatus for ElectronicModules.” The contents of the above applications are incorporated byreference.

TECHNICAL FIELD

This invention relates to over-molded packages for electronic modulessuch as power converter modules that include inductive components suchas inductors and transformers.

BACKGROUND

An encapsulated electronic module, such as an electronic power convertermodule for example, may comprise a printed circuit assembly over-moldedwith an encapsulant to form some or all of the package and exteriorstructure or surfaces of the module. Encapsulation in this manner mayaid in conducting heat out of the over-molded components, i.e.,components that are mounted on the printed circuit assembly and coveredwith encapsulant. In the case of an electronic power converter module,the printed circuit assembly may include one or more inductivecomponents, such as inductors and transformers. Encapsulated electronicpower converters are described in Vinciarelli et al., Power ConverterPackage and Thermal Management, U.S. Pat. No. 7,361,844, issued Apr. 22,2008, assigned to VLT, Inc. of Sunnyvale, Calif. and incorporated byreference in its entirety (the “Converter Package Patent”).

Methods of over-molding both sides of a printed circuit board assemblywhile leaving opposing regions on both sides of the printed circuitboard free of encapsulant are described in Saxelby, et al., CircuitEncapsulation Process, U.S. Pat. No. 5,728,600, issued Mar. 17, 1998 andSaxelby, et al., Circuit Encapsulation, U.S. Pat. No. 6,403,009, issuedJun. 11, 2002 (collectively the “Molding Patents”) (both assigned toVLT, Inc. of Sunnyvale, Calif. and incorporated by reference in theirentirety).

Protecting an over-molded permeable magnetic component from mechanicalstress by use of a compliant buffer coating is described in Lofti et al,U.S. Pat. No. 5,787,569, “Encapsulating Package for Power MagneticDevices and Method of Manufacture Thereof” Combining an un-encapsulatedpermeable magnetic component and an over-molded circuit assembly isdescribed by Vinciarelli et al, in Power Converter Configuration,Control and Construction, U.S. Pat. No. 7,236,086, issued Jun. 26, 2007;and Power Converter Having Magnetically Coupled Control, U.S. Pat. No.6,208,531, issued Mar. 27, 2001; both assigned to VLT, Inc. ofSunnyvale, Calif. An inductive charger including a permeable magneticcomponent in which a surface of the magnetic component may be exposedafter over-molding is described in Abbott et al, Inductive Coupling WandHaving a Molded Magnetic Core, U.S. Pat. No. 5,719,483.

SUMMARY

In one aspect, in general, an electronic module includes an electronicassembly that has electronic circuitry, a contact structure for makingelectrical connections to the circuitry, and a core structure fordirecting a magnetic field along a flux path, the core structure havinga core surface. The electronic module includes encapsulation materialsurrounding portions of the electronic assembly and forming an externalsurface of the electronic module. A first portion of the core surface issubstantially parallel to the flux path and exposed within the externalsurface.

Implementations of the electronic module can include one or more of thefollowing features. The core structure can include a magneticallypermeable material having a permeability to define the flux path.

The core structure can include gaps in the permeability along the fluxpath, the gaps can be encapsulated within the module, and the exposedfirst portion of the core surface can be free of gaps. The electronicassembly can include a printed circuit board (“PCB”) having a first areaincluding traces forming part of at least one winding. The corestructure can include an internal space having an internal core surfacesurrounding the first area of the PCB, the internal core surface beingseparated from the first area by a predetermined minimum distance filledwith encapsulant. The core structure can include a path thicknessperpendicular to the flux path in a direction radially outward from theinternal perimeter of the flux, and a minimum radius along the internalperimeter of the flux path, the minimum radius being at least 15% of thepath thickness. The minimum radius can be at least 15% of the paththickness. The external surface can include a second flat area separatedby a distance from the first flat area, wherein the distance is normalto the first flat area. The core surface can include a second portionsubstantially parallel to the flux path and separated from the exposedfirst portion by a distance normal to the exposed first portion, and thesecond portion of the core can be exposed within the second flat area.The minimum radius can be at least 25% of the path thickness.

The exposed first portion of the core surface can be substantially flat.The external surface can include a first flat area and the exposed firstportion of the core surface can be exposed within and coplanar with thefirst flat area. The first flat area can form a top surface of theelectronic module. The apparatus can include a controlled dimensionbetween the exposed first portion of the core and the contact structure.The contact structure can include a contact surface parallel to andbelow the first flat area and separated by the controlled distance fromthe first flat area.

The external surface can include a second flat area separated by adistance from the first flat area, wherein the distance is normal to thefirst flat area. The core surface can include a second portionsubstantially parallel to the flux path and separated from the exposedfirst portion by a distance normal to the exposed first portion. Thesecond portion of the core can be exposed within the second flat area.In some examples, the exposed second portion can be flat and recessedfrom the second flat area. In some examples, the exposed second portioncan be flat and coplanar with the second flat area. The exposed secondportion can protrude from the second flat area.

The external surface can include a second flat area separated by adistance from the first flat area, wherein the distance can be normal tothe first flat area. The core surface can include a second portionsubstantially parallel to the flux path and separated from the exposedfirst portion by a distance normal to the exposed first portion. Thesecond portion of the core can be encapsulated within the module beneaththe second flat area.

The exposed first portion of the core surface can be free of gaps inmagnetic permeability.

The electronic module can be a self-contained switching power converterhaving an input for receiving electrical energy from a source and anoutput for supplying electrical energy to a load.

In another aspect, in general, a method of forming an encapsulatedelectronic device includes providing an electronic assembly including afirst component having a first structure moveable with respect to asecond structure, providing a mold for encapsulating the electronicassembly and for forming at least a portion of an exterior shape of theencapsulated electronic device with cured encapsulant, closing a moldaround at least a portion of the electronic assembly, positioning thefirst structure relative to the second structure, and filling the moldwith an encapsulating material.

Implementations of the method can include one or more of the followingfeatures. The positioning can include clamping the second structurerelative to the mold and biasing the first structure in position againsta predetermined feature of the mold. The predetermined feature can be aninner surface of the mold. The biasing can include using a compliant padbetween the mold and the first structure to apply compliant pressure tothe first structure. The compliant pad can be affixed to the firststructure before closing the mold around the electronic assembly. Thecompliant pad can be affixed to a sheet, and the method can furtherinclude placing the sheet in the mold before closing the mold around theelectronic assembly. The method can include providing a moveable insertin a portion of the mold, the moveable insert having an insert surfacefor engaging and applying pressure to the first structure. The secondstructure can include a printed circuit board (“PCB”) and the firststructure can include a core structure for directing a magnetic fieldalong a flux path, the core structure having a first core surface andwherein the predetermined feature is a first inner surface of the mold.

The first inner surface of the mold can be contoured to produce aresulting flat exterior top surface for the electronic device afterencapsulation. The first core surface can be essentially flat and thepositioning can include moving the first core surface against the firstinner surface forming a seal to leave the first core surface exposedwithin and coplanar with the resulting flat exterior top surface. Themold insert can be provided in a bottom portion of the mold, adapted topush the core structure upward against the first inner surface. Theinsert surface can be adapted to form a seal against a second surface ofthe core structure to leave the second surface of the core structureexposed within a resulting exterior bottom surface of the device.

The mold insert can protrude from a bottom interior surface of the moldto produce a recess in the resulting exterior bottom surface of theelectronic device after encapsulation and the core structure can beexposed within the recess. The method can include adapting the corestructure to move over a range relative to the PCB, and providing aminimum clearance between the core structure and the PCB over the range.The encapsulated electronic device can include a power converter.

In another aspect, in general, an apparatus includes a core structurehaving a first core piece having a back section and a first leg fordirecting a first magnetic field along a flux path in the core. The backsection can have an exterior back surface and an interior back surfaceseparated by a back thickness, the back thickness being generally normalto the first flux path. The first leg can have an exterior leg surfaceand an interior leg surface separated by a leg thickness, the legthickness being generally normal to the flux path. The interior backsurface and the interior leg surface can include a bevel having a radiusgreater than or equal to 15% of the back thickness.

Implementations of the apparatus can include one or more of thefollowing features. The bevel can be rounded. The bevel can include oneor more segments approximating a rounded surface. The first core piececan further include a second leg having an exterior leg surface and aninterior leg surface separated by a leg thickness, the leg thicknessbeing generally normal to the flux path, and the interior back surfaceand the interior leg surface of the second leg can be joined by a bevelhaving a radius greater than or equal to 15% of the back thickness. Insome examples, one or more of the bevels can have a radius greater thanor equal to 25% of the back thickness. In some examples, one or more ofthe bevels can have a radius greater than or equal to 35% of the backthickness. The core structure can further include a second core pieceadapted to mate with the first and second legs of the first core piecefor directing the magnetic field along a flux path that includes inseries the first leg, the back section, the second leg, and the secondcore piece. The core structure can further include a second core piecehaving a back section, a first leg, and a second leg, for directing themagnetic field along a flux path in the core. The back section of thesecond core piece can have an exterior back surface and an interior backsurface separated by a back thickness generally normal to the flux path.The first and second legs of the second core piece each can have anexterior leg surface and an interior leg surface separated by a legthickness, the leg thickness being generally normal to the flux path.The interior back surface and the interior leg surfaces of the first andsecond legs of the second core piece can include bevels having a radiusgreater than or equal to 15% of the back thickness. The first and secondcore pieces can be adapted to mate together with the first and secondlegs meeting at first and second interfaces.

In some examples, one or more of the bevels can include a radius greaterthan or equal to 25% of the back thickness. In some examples, one ormore of the bevels can include a radius greater than or equal to 35% ofthe back thickness. The first and second core pieces each can include acenter leg between the first and second legs, each center leg havinginterior leg surfaces separated by a center leg thickness, each interiorleg surface being connected to the interior back surface by a bevelhaving a radius greater than or equal to 15% of the back thickness, andwherein the center legs meet at a center interface. At least one of theinterfaces can include a gap in magnetic permeability. The exterior backsurface can be essentially flat. The first and second core pieces caninclude a magnetically permeable material. The exterior of the corestructure along an outside perimeter of the flux path can include agenerally rectangular shape. The leg thicknesses can be approximatelyequal to the back thicknesses, and the center leg thicknesses can beapproximately double the back thickness.

In some examples, the bevels can include a radius greater than or equalto 25% of the back thickness. In some examples, the bevels can include aradius greater than or equal to 35% of the back thickness. In someexamples, the bevels can include a radius greater than or equal to 50%of the back thickness. The flux path can include an interior perimeter,the interior perimeter of the flux path can include one or more bends,each bend including a radius, and each radius being greater than orequal to 25% of the back thickness. The flux path can include one ormore interior perimeters, each interior perimeter of the flux pathincluding one or more bends, each bend including a radius, and eachradius being greater than or equal to 25% of the back thickness.

In another aspect, in general, an apparatus includes a planar magneticcore structure including one or more loops of magnetically permeablematerial for directing a magnetic field along a flux path having aninner perimeter and an outer perimeter defined by its respective loop,the loop having a loop thickness defined by the distance between itsrespective inner perimeter and outer perimeter. The core structure has afirst generally flat exterior surface along and parallel to a firstsection of the outer perimeter of the flux path, wherein each innerperimeter includes one or more bends, each bend including a radius, andeach radius being greater than or equal to 15% of the loop thickness.

Implementations of the apparatus can include one or more of thefollowing features. The structure can further include a second generallyflat exterior surface along and parallel to a second section of theouter perimeter of the flux path, the second generally flat exteriorsurface being generally parallel to the first generally flat exteriorsurface. In some examples, each radius can be greater than or equal to25% of the loop thickness. In some examples, each radius can be greaterthan or equal to 35% of the loop thickness. The apparatus can furtherinclude power conversion circuitry including an inductive element havinga conductive winding that includes one or more turns encircling aportion of at least one of the one or more loops, wherein the turns havea conductor width and the conductor width is less than or equal to theloop thickness.

DESCRIPTION OF DRAWINGS

We first briefly describe the drawings:

FIG. 1 is an exploded perspective view of a printed circuit assembly.

FIGS. 2A and 2B are, respectively, top and bottom perspective views of aprinted circuit assembly.

FIG. 3 is a top perspective view of a prior art over-molded moduleassembly.

FIG. 4 is a bottom perspective view of a prior art over-molded moduleassembly.

FIG. 5 is a cross-sectional view of the assembly of FIGS. 3 and 4.

FIG. 6 is a top perspective view of an over-molded module assemblyaccording to the invention.

FIG. 7 is a bottom perspective view of an over-molded module assemblyaccording to the invention.

FIGS. 8 and 9 are cross-sectional views of the over-molded moduleassembly of FIGS. 6 and 7 respectively taken along lines A-A and B-B inFIG. 6.

FIGS. 10 and 11 are exploded perspective views of an apparatus formolding a module according to the invention.

FIGS. 12 and 13A-C are cross-sectional views of the apparatus of FIGS.10 and 11 in operation.

FIGS. 14 and 15 are exploded perspective views of second apparatus formolding a module according to the invention.

FIGS. 16 and 17 are exploded perspective views of a third apparatus formolding a module according to the invention.

FIGS. 18A and 18B are respectively exploded perspective andcross-sectional views of an improved core structure.

FIGS. 19A and 19B are respectively exploded perspective andcross-sectional views of a second improved core structure.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

An example of a printed circuit board assembly 50, such as a powerconverter, is shown prior to encapsulation in FIGS. 1, 2A, and 2B. Inthe exploded perspective view of FIG. 1, the printed circuit boardassembly 50 is shown including a printed circuit board (“PCB”) 10 havinga top surface 12 and a bottom surface 14 and active and passivecomponents such as components 17 and 19. The PCB assembly 50 is shownincluding an inductive component, which may be a transformer, having topand bottom magnetically permeable core pieces 20, 22 and one or morewindings. The windings may be formed from conductive traces on one ormore layers of PCB 10. One such winding 16 is illustrated in FIG. 1. Foruse in an encapsulated assembly, the permeable core pieces 20, 22 mayhave flat grooved top and bottom surfaces 34, 36 respectively as shownin FIG. 1.

As illustrated in FIG. 1, the inductive component may be configured byinstalling the magnetically permeable core pieces 20, 22 into slots 24a, 24 b, 24 c formed in the PCB 10 with opposing pairs of core faces 31a, 31 b; 32 a, 32 b; 33 a, 33 b passing through the slots. Cores of thetype shown in FIG. 1 are known in the field of planar magnetics and arecommonly used in applications where size reduction is important.Opposing core faces may be arranged to either come in contact with eachother without a gap or to be in close proximity to each other so that agap is formed between the core faces. Referring to the top and bottomperspective views of FIGS. 2A and 2B, the PCB assembly 50 is shown withthe core pieces 20, 22 installed onto the PCB 10. As illustrated inFIGS. 2A and 2B, the inductive component that is formed in this way mayhave an essentially flat grooved top surface 34 that lies above the topsurface 12 of the PCB 10 and an essentially flat grooved bottom surface36 that lies below the bottom surface 14 of the PCB. Printed circuittransformers of this kind are described in Vinciarelli, Printed CircuitTransformer, U.S. Pat. No. 7,187,263, issued Mar. 6, 2007, assigned toVLT, Inc. of Sunnyvale, Calif. and incorporated by reference in itsentirety (the “Transformer Patent”).

Traditionally in encapsulated power converter assemblies, allcomponents, including the inductive components and their respective corepieces, have been over-molded, i.e. covered with encapsulant, forexample as shown in FIGS. 3 and 4. Covering all of the componentsincluding the magnetic core pieces provided a controlled thicknessdimension for the finished assembly while accommodating variations inthickness of the components, e.g. the core pieces. Additionally, thecore pieces were treated as components requiring encapsulation formechanical and electrical safety reasons. For example, enclosing thecores provided a uniform, co-planar top surface and a layer ofdielectric insulation over the cores.

FIGS. 3 and 4 respectively show top and bottom perspective views of aprior art package 100 described in the Converter Package Patent. Notethe distinction between the form of the package resulting fromencapsulation (shown in FIGS. 3-5) and the components (shown in FIGS. 1,2A, 2B, and 5) internal to the package. While the exterior shape andstructure of the over-molded package shown in FIGS. 3 and 4 may be foundin the prior art, the assembly being encapsulated and its componentsneed not be. Therefore reference to the package shown in FIGS. 3, 4, and5 as prior art is a reference to the encapsulation form and method forproducing that form, not the internal components.

The over-molded assembly 100 may be formed e.g. by encapsulating theprinted circuit assembly 50 of FIGS. 1 and 2 using an encapsulatingmaterial 40. As shown in FIGS. 3 and 4, the module 100 top surface 102and the module bottom surface 104 are formed entirely of curedencapsulating material 40. As shown, edges of the PCB 10 are exposedalong and thus form portions of the sides of the assembly's 100exterior. Referring to FIG. 5, the cross-section (taken along C-C inFIG. 3) of over-molded assembly 100 shows the encapsulated internal PCBassembly 50. As shown in the cross-section of FIG. 5, relatively thinwebs 41, 42 of encapsulating material 40 cover the top surface 134 ofcore piece 120 and the bottom surface 136 of core piece 122. Forming athin wall of encapsulant over the core may be problematic. For example,air voids may form due to inadequate mold flow, or blistering may occurdue to contaminants in the core, e.g. silicone used in the manufactureof ferrite cores. Certain permeable magnetic materials, such asferrites, are hygroscopic and may absorb moisture from the atmosphere.As a transformer heats up during operation, the entrapped moisture mayescape causing blistering of the encapsulant above the core. Theover-molded assembly 100 may therefore require baking the moisture outof the cores prior to encapsulation which can be costly in terms of bothtime and energy. Increasing the thickness of the encapsulant over thecores may be undesirable for the resulting increase in package thicknessand increase in thermal resistance through the encapsulant forelectronic components mounted on the PCB. Grooves 135 and bevels 138 maybe added to the core surfaces 134, 136 to minimize the area and increasethe physical integrity of the otherwise thin webs 41, 42 of encapsulant.Such features may increase the resistance against but do not eliminateblistering and void formation.

An improved over-molded electronic assembly 200 and an improved corestructure 210 is illustrated in FIGS. 6 through 9 and FIGS. 18A-18B,respectively. Module 200, like module 100 (FIGS. 3-5), may be formed byover-molding the electronic assembly with an encapsulating material 40.Unlike module 100, the core surfaces 234, 236 are exposed in theimproved assembly 200, providing potential improvements in thermalperformance, efficiency, reliability, and manufacturing yield asdiscussed in more detail below. As shown, the exposed core assembly 200includes a PCB assembly (similar to that described above with referenceto FIGS. 1 and 2) including PCB 10 but with an improved core structure210 (FIG. 18A-B) described below. Other selections, configurations, andarrangements of components or PCB assemblies may be used in the exposedcore package 200.

Referring to the FIG. 5, which shows a prior art planar magnetic core incross section, the windings (16 in FIG. 1) on PCB 10 extend, andtherefore current flows, into and out of the page setting up a magneticfield with a flux path encircling the openings 160. Note that the priorart planar magnetic structure has relatively sharp, small-radii cornersalong the inside perimeter of the magnetic path creating substantiallyrectangular openings 160 in the core assembly. These essentially squaredcorners, ubiquitous in planar magnetic structures, help to maximize thespace available for the PCB windings and clearance to the core for agiven core size. The interior corners of many planar magnetic structuresare also made to include a trench around the core legs, i.e., a negativeradius at the corner, to ensure that any interior bevels are removedfrom the opening, e.g. to ensure a tight fit with the PCB. Although thesharp corners allow for larger windings, with the objective of reducingwinding resistance, they can cause a concentration of flux around thecorners. The concentration of flux can create localized hot spots in thepermeable material due to saturation at high frequency. Ironically,planar cores should be made to enable higher operating frequencies toreduce the size of power converters.

An improved high-frequency planar core configuration is illustrated inFIGS. 19A-19B. In the exploded perspective view of FIG. 19A, two matingcore halves 520, 522 are shown separated and facing each other. Across-section of the assembled core 510 is shown in FIG. 19B. The corepieces may be assembled together separated by a gap 400 which preferablymay be divided between the center and outer legs. The gaps 400 may beformed in various ways, including the use of flat shims or balls offixed diameter suspended in an adhesive between the core pieces.Depending upon the specific requirements, the gap 400 may be increased,decreased, or eliminated completely.

As shown in FIG. 19B, the cores form two loops of permeable material fordirecting magnetic fields within the core. Each loop surrounds arespective opening 560 and includes two respective air gaps 400. Notethat core 510 is constructed for use with the same winding orientationshown in FIG. 1, extending, and thus current flowing, into and out ofthe page in the cross section of FIG. 19B. The flux path of theresulting magnetic field is in the plane of the page surrounding theopenings 560 in the cross-section of the core 510. The cross-section ofthe core 510 as shown in FIG. 19B approximates two end-to-end oblongtoroids. As shown, the core 510 includes a substantially uniformthickness A3, where the thickness may be defined in the directionperpendicular to the magnetic flux path through the permeable material,e.g. from the inner perimeter to the outer perimeter of the magneticpath. As revealed by the shape of the openings 560, the core includeslarge radii bends 565 along the inner perimeter and a rounding of theexterior edges along the outer perimeter of the magnetic path throughthe permeable core.

Referring to FIGS. 18A and 18B, a modified improved high-frequencyplanar core structure 210 adapted for use in the exposed core package200 is shown. In the exploded perspective view of FIG. 18A, two matchingcore halves 220, 222 are shown separated and facing each other. The corepieces may be assembled together with or without a gap 400 as discussedabove in connection with FIG. 19. As shown in the cross-section of FIG.18B, the assembled core 210 forms a magnetically permeable path withgaps 400, preferably divided between the center and outer legs. Themagnetic path surrounds two openings 260, which, like the core 510 ofFIG. 19, includes large inner radii bevels or bends 265 along the innerperimeter of the magnetic flux paths.

The radius of the bends 265, 565 are a substantial fraction of the corethickness A2, A3, where the thickness may be defined in the directionperpendicular to the magnetic flux path through the core, e.g. from theinner perimeter to the outer perimeter of the magnetic path. Forexample, assuming the magnetic path has a thickness A2 (FIG. 18B) or A3(FIG. 19B) the minimum radius of bevels 265 or 565 respectively may begreater than or equal to 15% and may be beneficially increased to 25%,and preferably 35%, or even 50% of the thickness A2, A3 of themagnetically permeable path to help reduce concentration of the magneticfield around bends along the inner perimeter of the magnetic path andthe losses associated with it. The large radii bends 265, 565 of planarcores 210, 510 along the inner perimeters of the magnetic flux path helpreduce or eliminate concentration of the magnetic field around thebends, increasing efficiency. Increases in the internal radii beyond 35%to 50% may overly reduce the amount of space available for the windingsin any given core and increase the volume of the core causing increasedcore loss. As discussed in more detail below, the large radii bends incombination with other improvements enable higher frequency converteroperation, reduction in converter size, increase in converter powerdensity, faster dynamic response, and increase in efficiency of thecore. Unlike the more idealized core 510 (FIG. 19B), core 210 (FIG. 18B)has substantially flat top 234 and bottom 236 surfaces which make itparticularly amenable for use in the exposed core package 200 discussedin detail below. The squaring of the core along the outside perimeter ofthe magnetic path has a minimal negative impact on flux crowding andefficiency compared with squaring of the inside corners of the corealong the inner perimeter of the flux path.

Although the rounded openings 260, 560 in the core reduce the areaavailable for the windings, the width of the winding conductors isreduced in FIG. 9 rather than using larger openings. Perhapscounter-intuitive because of the increase in DC resistance in thewindings, narrowing the windings can have a net effect of increasing theoverall efficiency of converters in which the magnetic structure is usedas an inductor or flyback transformer. Using narrower winding conductorsincreasing the separation of the conductors from the gaps 400 helpsreduce high frequency eddy current losses induced in the windings by thefringing fields. Splitting the gap between the center and outer legs asshown helps not only reduce the fringing fields by shortening each gapby a factor of two, but also helps to better distribute the currentdensity by attracting current to the outside edges of the windings inaddition to the inside edges. The width of the windings may be reducedsignificantly without incurring an efficiency penalty. Winding widths assmall as half of the core thickness may be used in the cores 210, 510 ofFIGS. 18A, 18B, 19A and 19B. Since the current distribution in thewinding conductors at high frequencies is in the edges near the gaps,the increase in DC resistance may be more than offset by the reductionin losses due to eddy currents and reduction in flux concentration atthe bends along the inner perimeter of the magnetic flux paths. Thus,the improved planar magnetic structures 510, 210 of FIGS. 19A, 19B and18A, 18B respectively may be used to improve the efficiency of the coreand of the windings. For example, the core configuration 210 shown inFIG. 18A-B may achieve as much as a 20% improvement in efficiency overthe prior art planar core shown in FIGS. 1-5 with comparable core andleg thickness.

Referring to FIGS. 6 (top perspective view), 8 (cross-section along A-Ain FIG. 6), and 9 (cross-section along B-B in FIG. 6) which show themodule 200 after encapsulation, the outer surface 234 of core piece 220is shown exposed, i.e. not covered with encapsulating material 40. Theouter surface 234 of core piece 220 is shown generally coplanar with theessentially flat top surface 202 of the over-molded assembly 200.Referring to FIG. 5 for contrast, the outer surface 134 of core piece120 in the prior art package 100 is shown over-molded, i.e. covered witha thickness of encapsulating material 40. The outer surface 236 of corepiece 222 may also be exposed as shown in FIGS. 7 (bottom perspectiveview), 8 (cross-section along A-A in FIG. 6), and 9 (cross-section alongB-B in FIG. 6). Core surface 236 may be exposed within a recess 206 inthe molded lower surface 204 as shown in FIGS. 7, 8, and 9 for exampleto ensure encapsulation of other components protruding from the bottomsurface 14 of PCB 10. Alternatively, the depth of the recess 206 may bereduced to zero allowing core surface 236 to be coplanar with the moldedlower surface 204 like core surface 234 which is coplanar with moldedupper surface 202.

Keeping the core surfaces exposed reduces the thermal resistance betweenthe core and the surface of the assembly, potentially improving heatremoval. With reference to FIGS. 5 and 9, assuming the thickness of thetransformer (D1, FIG. 5; D2, FIG. 9) is fixed (i.e., D1=D2), eliminationof over-molding on the top of the transformer may allow the overallthickness, T2, of the assembly 200 to be reduced relative to thethickness T1 of the prior art assembly 100. The reduction in the overallthickness of the over-molding material between the other internalcomponents, such as components 17, 19 (FIG. 1) and the outer surfaces ofthe module 200 may reduce the thermal resistance along that path andthereby reduce the operating temperature of the individual componentsand the assembly overall.

With reference to FIGS. 5 and 9, assuming the thickness of the moduleassembly (T1, FIG. 5; T2, FIG. 9) is fixed (i.e., T1=T2), elimination ofthe encapsulant 41 over the transformer may allow the overall thicknessD2 of the transformer core pieces 220, 222 in assembly 200 to beincreased relative to the thickness, D1, of the transformer core pieces120, 122 in the prior art assembly 100. For an established set ofoperating conditions, the increased thickness D2 may allow for thickercore pieces, e.g. dimension A2 (FIG. 9) may be greater than A1 (FIG. 5),increasing the cross-sectional area and reducing flux density andultimately reducing core losses. Assuming for example that the prior artpower converter module has an overall thickness T1=6.5 mm and atransformer thickness D1=5 mm. Elimination of the 1 mm of totalover-molding above and below the transformer may allow the transformerthickness to be increased to D2=6 mm, representing a 20% improvement.Because core loss is an exponential function of flux density (e.g., coreloss is related to the flux density raised to the 2.9^(th) power), a 20%increase in cross section may result in a nearly 50% reduction in corelosses. Increased core cross-section may also reduce fringing fields andeddy current losses.

The exposed core package 200 illustrated in FIGS. 6 through 9 may allowtrapped moisture in the core to escape during operation withoutblistering, potentially eliminating the need for and the cost of bakingthe cores prior to encapsulation. It may also reduce mechanical stressesplaced on the core potentially eliminating the need for buffer coatingsand further reducing core losses. The exposed core package 200eliminates the thin layer of encapsulation over the core and theproblems associated with those layers, including void formation,increases in thermal resistance, and cosmetic defects.

Referring to FIG. 9, top 261, bottom 262, and lateral 263 interstitialgaps are created between the PCB 10 and the cores 220, 222, in theinterior spaces 260 (FIG. 18B) formed between the cores. As shown inFIG. 9, lateral exterior gaps 264 are also formed between the cores andthe PCB 10. The respective size of gaps 261, 262, 263, and 264 may beset to establish a minimum clearance between the core and the PCB(containing the windings) and allow encapsulate to fill those spacesduring the molding process. The molding compound forms an insulatingwall providing safety insulation between the primary winding and thecore. In this way predetermined minimum safety spacing and dielectricinsulation are provided between the core 210 and the PCB 10 ensuringthat the core is properly isolated from the windings, allowing the coreto be exposed and grounded or tied to the secondary winding if desired.

As illustrated in FIG. 9, the top 220 and bottom 222 E-shaped cores arefashioned to establish interior spaces 260 that are greater in heightthan the maximum thickness of the PCB 10 and the minimum gap sizerequired for the top 261 and bottom 262 interstitial gaps. Compare forexample, the difference in the gaps 161 and 162 of module 100 (FIG. 5)with relatively larger gaps 261 and 262 of module 200 (FIG. 9) and inthe height F1 of the interior spaces of module 100 (FIG. 5) with therelatively larger height F2 of the interior spaces 260 of module 200(FIG. 9). The minimum height F2 of interior space 260 is set to allowthe assembled core 210 to move vertically relative to the PCB 10 whichmay be used during the manufacturing process to adjust the core surface234 into a precise position relative to the PCB for encapsulation.

For example, the core may be moved upward into a position that will becoplanar with the top surface 202 of encapsulated module 200, i.e. incontact with the top inner surface of a mold cavity. The minimum heightF2 may be set to accommodate the requisite movement, e.g. to adjust forvariations in the PCB 10 thickness and core 210 dimensions, whileensuring that sufficient space remains for top and bottom interstitialgaps 261, 262, e.g. for safety and encapsulation clearances, to allowthe core 210 to be positioned, e.g. a controlled distance from terminalslocated on the bottom surface 14 of PCB 10 or in precise relation to thetop surface 202, etc.

A molding apparatus 300 for encapsulating a PCB assembly 50 in a mannerthat produces the exposed core package 200 is shown schematically inFIGS. 10 through 13. FIGS. 10 and 11 show exploded perspective views ofthe molding apparatus 300 from above and below, respectively. FIGS. 12and 13 show cross-sectional views through the apparatus 300 with themold closed and in operation.

Referring to FIGS. 10 through 13C, the molding apparatus 300 may includea top mold section 310 having a recessed interior surface 314 contouredto establish the shape of the top exterior surface of the assembly 200after the molding process. A lower base 302 may support a bottom moldsection 308 and a mold insert 306, which may be biased upward by springs304. As shown, the bottom mold section 308 includes a top surface 321, arecessed interior surface 322, and an aperture 312. The aperture 312 mayhave a shape that matches the insert to form a seal while allowing theinsert 306 to move up and down in the aperture under pressure from thesprings 304. As illustrated, the surface of the recess 322 is contouredand has a shape that together with the top surface 330 of insert 306establishes the shape of the bottom exterior surface of the assembly 200after the molding process.

FIGS. 10 and 11 show an example of an exposed core module 200 (afterencapsulation and singulation) in exploded relation to the moldingapparatus 300. Note that in FIG. 10, the outer surface 234 of core piece220 is shown exposed and coplanar with the top surface 202 of theexposed core module 200. In contrast, the outer surface 236 of corepiece 222 is shown exposed within a recess 206 in the bottom surface 204of the exposed core module 200 in FIG. 11. FIG. 12 is a cross-sectionalview of the molding apparatus 300 through section G-G (FIG. 10) with thetop 310 and bottom 308 molds closed against the PCB 10 of the PCBassembly and filled with an encapsulant. FIG. 13A through 13C arecross-sectional views of the molding apparatus 300 and PCB assembly atvarious stages during the molding process through section H-H (FIG. 10).

In operation, the top 310 and bottom 308 molds may be closed against thePCB assembly (similar to the PCB assembly 50 in FIGS. 1 and 2) eachforming a seal against the PCB 10 as shown in FIGS. 12 and 13A-13C forexample as described in the Molding Patents. The portions of the PCBagainst which the molds form a seal may be trimmed after the moldingprocess. Alternatively, the molds may be configured to seal against eachother with the PCB enclosed within the mold cavity or a combination ofthe two approaches in which portions of the mold cavity perimeter aresealed directly between the molds and other portions are sealed with thePCB 10 sandwiched between the top and bottom molds may be used.

As discussed above, the configuration of the core pieces is such thatthe assembled core may move up and down relative to the PCB 10. As thetop mold section 310 and bottom mold section 308 are closed against thesurfaces of PCB 10 along its outer periphery, springs 304 bias insert306 upward within aperture 312, forcing the flat upper surface 330 ofinsert 306 into contact with flat surface 236 on core piece 222. Underthe influence of the spring loaded insert 306, the core is moved upwardwith the flat upper surface 234 of core piece 220 forced into contactwith flat interior surface 314 of the top mold section 310.

The sequence of FIGS. 13A through 13C illustrate movement of the moldinsert 306 and the core relative to the PCB 10 under bias from springs304 into position with core surface 234 in contact with the innersurface 314 of top mold 310. In FIG. 13A, the insert 306 and the coreassembly 210 are shown in a lowered position relative to PCB 10 and topmold 310. With the core assembly 210 in a lowered position, the PCB 10is at the top of the interior spaces 260 with the top interstitial gaps261 minimized and the bottom interstitial gaps 262 maximized and thereis a gap 315 between the inner surface 314 of the top mold and the coresurface 234. (Although FIG. 13A shows the insert and core in a loweredposition with the mold closed against the PCB 10, in practice the insertand core may be moved into the final position as the mold is closedagainst the PCB 10 rather than afterward as illustrated.) In FIG. 13B,the springs 304 have forced the insert upward moving the core intoposition with core surface 234 against the inner surface 314 of top mold310 eliminating the gap 315. The interstitial gaps 261 and 262 are shownmore balanced in FIG. 13B. With the core in position, FIG. 13C shows thetop 310 and bottom 308 molds filled with an encapsulant 40.

After the mold is closed, encapsulant is forced into the mold, fillingall empty spaces within the mold, including the top 261, bottom 262,interstitial side 263, and exterior side 264 gaps between the corepieces 220, 222 and the PCB 10 (FIG. 9). Because core surfaces 234 and236 are in respective contact with mold surfaces 314 and 330, theyremain free of over-molding and exposed in the final encapsulatedassembly 200.

PCB assemblies having identical exterior package shapes may be adaptedto different operating configurations. Consider for example, a series ofpower converters (with the same package) having different inputvoltages, output voltages, and power ratings. Such variations inoperating configurations may require inductive components, such astransformers, that differ in properties and further requiring corepieces that differ in, e.g. thicknesses, in core gaps 400, and inconstruction. Variations in the core gaps 400 or in the thickness A2 ofthe cores or the length of the core legs may cause variations in thethickness dimension (e.g., thickness D2, FIGS. 8, 9, 18) of theassembled core 210. It will be evident from the above description thatsuch variations in thickness (e.g., due to tolerances within aparticular converter model or due to parametric differences betweenconverter models in the series) may be accommodated by a single generalpurpose molding apparatus. The core 210 of each assembly is configuredwith the requisite minimum height F2 to be positioned properly forencapsulation and ensure the minimum safety clearance. The top coresurface 234 may be positioned flush with the top surface 202 of theencapsulated assembly 200 with the bottom core surface 236 recessed toaccommodate the variations. Referring to FIG. 12, the overall thicknessof the core assembly (including the gap 400 if any) will determine theposition of insert 306 during encapsulation and thus the depth of theresulting recess 206. Variations in the thickness D2 of the assembledcore 210 may thus be accommodated by the movable mold insert 306.

Depending upon the thickness D2 of the core and the thickness T2 of theencapsulated exposed core assembly 200, the exposed core surface 236 maybe recessed within the encapsulated assembly 200 as shown in FIGS. 10through 13; or for thicker cores or thinner encapsulated assemblies 200,the exposed core surface 236 may be coplanar with surface 204.Alternatively, if desired, still thicker cores and thinner encapsulatedassemblies may be accommodated by allowing the exposed core surface toprotrude from surface 204 in which case the insert 306 would bedepressed within the aperture 312 with the insert surface 330 below themold surface 322.

Referring to the exploded views of FIGS. 14-15 an alternative moldingapparatus 340 for making the exposed core assembly 200 is illustrated.As shown the lower mold 308 including the aperture 312, springs 304, andinsert 306 of molding apparatus 300 (FIGS. 10-13C) have been replaced bya simplified lower mold 348. Lower mold 348 contains an inner contour342 for forming the lower exterior surface of the exposed core assembly200. However instead of using the moveable insert 306 under the force ofsprings 304 to move the core 210 into position for encapsulation (FIGS.10-13C), a compliant pad 341, e.g. made from silicon rubber, may beused. The compliant pad 341 may be sized to match and pre-assembled ontothe bottom surface 236 of the core 210 prior to insertion into themolding apparatus 340. As the mold 340 is closed on the PCB assembly,the compliant pad biases the core 210 upward against the inner surfaceof top mold 310 and compresses to absorb any dimensional deviationsamong cores. A predetermined amount of compression in the pad ensuresthat the encapsulant does not cover lower core surface 236. Thethickness of the compliant pad 341 may be selected to accommodate coreassemblies having different thicknesses D2 (FIG. 9). For example, athinner core may use a thicker pad and vice versa. Using the compliantpad, the molding apparatus 340 produces the exposed core package 210with core surface 234 exposed and coplanar with the surface 202 and coresurface 236 exposed within a recess 206 in surface 204.

An alternative molding apparatus 350 is shown in the exploded views ofFIGS. 16 and 17. Like apparatus 340 in FIG. 14-15, a lower mold 358(similar to lower mold 348) is used in place of the lower mold 308,aperture 312, springs 304, and insert 306 used in apparatus 300 of FIGS.10-13C. In apparatus 350, however, the compliant pad 351 is attached toa thin carrier film 352 that registers with lower mold 358 to positionthe pad 351. The depth of the cavity in lower mold 358 may be increasedrelative to lower mold 348 to accommodate the thickness of the carrierfilm.

The molding apparatus 340 and 350 do not require movable inserts whichallows assemblies with or without cores to be molded in the sameapparatus. Additionally, PCB assemblies having different quantity andlocations of cores may be encapsulated without changing the molds.

The benefits of the exposed core package 200 and the improved core 210(FIGS. 9, 18A, 18B) may be illustrated by comparing the characteristicsof two similar power converter modules having the same overall moduledimensions. The “baseline” converter includes the prior art core andencapsulation shown in FIGS. 1-5. The “improved” version includes theimproved core 210 (FIGS. 9, 18A, 18B) and exposed core package 200(FIGS. 6-9) and will, otherwise, have a power train similar to thebaseline version.

The improved converter has the following dimensions (with reference toFIG. 9): module thickness, T2=6.4 mm; core thickness D2=5.7 mm; magneticpath thickness, A2=1.7 mm; side leg thickness, C2=1.7 mm (same as themagnetic path thickness); center leg thickness, G2=3.4 mm; internalradii=0.64 mm (approx. 37% of the path thickness A2); opening width,B2=3.25 mm; overall core width=13.3 mm; and a winding-conductor width,E2=0.9 mm (note that FIG. 9 does not show the setback of the conductorfrom the PCB edge). The “baseline” converter has the followingdimensions (with respect to FIG. 5): module thickness, T1=6.4 mm; corethickness D1=5.0 mm; magnetic path thickness, A1=1.42 mm; side legthickness, C1=1.42 mm (same as the magnetic path thickness); center legthickness=2.95 mm; internal radii=0.2 mm (approx. 14% of the paththickness A1); opening width, B1=3.76 mm; overall core width=13.3 mm;and a winding-conductor width, E1=1.8 mm (note that FIG. 5 does not showdetails of setback of the conductor from the PCB edge). Note that theconverter used as the baseline for the comparison is not purely priorart because it used a modified core having bends with a positive radiusof approximately 14% of the magnetic path thickness, rather thanessentially square or negative radius bends. For that reason, thecomparison yields somewhat understated, but still illustrative, results.

The improved planar magnetic core structure of the improved convertershowed a 45% reduction in core loss at 500 W of converter powerthroughput compared to the planar magnetic core structure of thebaseline converter.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. For example, themovable insert may define the entire bottom surface of the encapsulatedassembly to avoid formation of the recess 206, allowing the overallthickness T2 (FIG. 9) of the encapsulated assembly to be defined by thethickness D2 of the core. A multi-cavity mold may include several moldapparatus 350 over which a unitary thermal film having multiple pocketswith multiple compliant pads could be placed to mold several assembliesat the same time. Pins or other features may be provided in the mold tobias the core upward producing an assembly in which the top core surfaceis exposed and the bottom core surface is covered with encapsulant.Alternatively, fluid dynamics may be used to bias the core upwardagainst the top mold. Advantageous results may be achieved if the stepsof the disclosed techniques were performed in a modified sequence, ifcomponents in the disclosed systems were combined in a different manner,or if the described components were replaced or supplemented by othercomponents. The disclosed systems or certain sub-components may beintegrated, in whole or in part. Accordingly, other embodiments arewithin the scope of the following claims.

What is claimed is:
 1. A method of forming an encapsulated electronicdevice comprising: providing an electronic assembly including one ormore first components, each first component having a first structure anda second structure, the first structure moveable with respect to thesecond structure; providing a mold for encapsulating the electronicassembly including the one or more first components, and for forming atleast a portion of an exterior shape of the encapsulated electronicdevice with cured encapsulant; closing the mold around at least aportion of the electronic assembly including the one or more firstcomponents; for at least one of the one or more first components,positioning the first structure of the first component relative to thesecond structure of the first component within a cavity of the mold; andfilling the cavity of the mold with an encapsulating material tosurround the portion of the electronic assembly including the one ormore first components.
 2. The method of claim 1 wherein the positioningcomprises clamping the second structure relative to the mold and biasingthe first structure in position against a predetermined feature of themold.
 3. The method of claim 2 wherein the predetermined feature is aninner surface of the mold.
 4. The method of claim 2 wherein the biasingcomprises using a compliant pad between the mold and the first structureto apply compliant pressure to the first structure.
 5. The method ofclaim 4 wherein the compliant pad is affixed to the first structurebefore closing the mold around the electronic assembly.
 6. The method ofclaim 4 wherein the compliant pad is affixed to a sheet and furthercomprising placing the sheet in the mold before closing the mold aroundthe electronic assembly.
 7. The method of claim 2 further comprisingproviding a moveable insert in a portion of the mold, the moveableinsert having an insert surface for engaging and applying pressure tothe first structure.
 8. The method of claim 7 wherein the secondstructure comprises a printed circuit board (“PCB”) and the firststructure comprises a core structure for directing a magnetic fieldalong a flux path, the core structure having a first core surface andwherein the predetermined feature is a first inner surface of the mold.9. The method of claim 8 wherein: the first inner surface of the mold iscontoured to produce a resulting flat exterior top surface for theelectronic device after encapsulation, the first core surface isessentially flat, and the positioning comprises moving the first coresurface against the first inner surface forming a seal to leave thefirst core surface exposed within and coplanar with the resulting flatexterior top surface.
 10. The method of claim 9 wherein the mold insertis provided in a bottom portion of the mold, adapted to push the corestructure upward against the first inner surface, the insert surface isadapted to form a seal against a second surface of the core structure toleave the second surface of the core structure exposed within aresulting exterior bottom surface of the device.
 11. The method of claim9 wherein the mold insert protrudes from a bottom interior surface ofthe mold to produce a recess in the resulting exterior bottom surface ofthe electronic device after encapsulation; and the core structure isexposed within the recess.
 12. The method of claim 11 further comprisingadapting the core structure to move over a range relative to the PCB;and providing a minimum clearance between the core structure and the PCBover the range.
 13. The method of claim 12 wherein the encapsulatedelectronic device comprises a power converter.
 14. The method of claim 1wherein one of the one or more first components comprises an inductivecomponent, the first structure comprises a magnetically permeable corestructure and the second structure comprises a printed circuit boardincluding a conductive winding magnetically coupled to the corestructure to form the inductive component.
 15. A method of forming anencapsulated electronic device comprising: providing an electronicassembly including a first component having a first structure moveablewith respect to a second structure; providing a mold for encapsulatingthe electronic assembly and for forming at least a portion of anexterior shape of the encapsulated electronic device with curedencapsulant; closing a mold around at least a portion of the electronicassembly; positioning the first structure relative to the secondstructure; and filling the mold with an encapsulating material, whereinthe second structure comprises a printed circuit board (“PCB”) and thefirst structure comprises a core structure for directing a magneticfield along a flux path, the core structure having a first core surfaceand wherein the predetermined feature is a first inner surface of themold.
 16. The method of claim 15 wherein: the first inner surface of themold is contoured to produce a resulting flat exterior top surface forthe electronic device after encapsulation, the first core surface isessentially flat, and the positioning comprises moving the first coresurface against the first inner surface forming a seal to leave thefirst core surface exposed within and coplanar with the resulting flatexterior top surface.
 17. The method of claim 16 wherein the mold insertis provided in a bottom portion of the mold, adapted to push the corestructure upward against the first inner surface, the insert surface isadapted to form a seal against a second surface of the core structure toleave the second surface of the core structure exposed within aresulting exterior bottom surface of the device.
 18. The method of claim16 wherein the mold insert protrudes from a bottom interior surface ofthe mold to produce a recess in the resulting exterior bottom surface ofthe electronic device after encapsulation; and the core structure isexposed within the recess.
 19. The method of claim 18 further comprisingadapting the core structure to move over a range relative to the PCB;and providing a minimum clearance between the core structure and the PCBover the range.
 20. The method of claim 19 wherein the encapsulatedelectronic device comprises a power converter.
 21. The method of claim16 wherein the positioning comprises clamping the second structurerelative to the mold and biasing the first structure in position againsta predetermined feature of the mold.
 22. The method of claim 21 whereinthe predetermined feature is an inner surface of the mold.
 23. Themethod of claim 21 wherein the biasing comprises using a compliant padbetween the mold and the first structure to apply compliant pressure tothe first structure.
 24. The method of claim 23 wherein the compliantpad is affixed to the first structure before closing the mold around theelectronic assembly.
 25. The method of claim 23 wherein the compliantpad is affixed to a sheet and further comprising placing the sheet inthe mold before closing the mold around the electronic assembly.
 26. Themethod of claim 21 further comprising providing a moveable insert in aportion of the mold, the moveable insert having an insert surface forengaging and applying pressure to the first structure.
 27. A method offorming an encapsulated electronic device comprising: providing anelectronic assembly including a first component having a first structureand a second structure, the first structure moveable with respect to thesecond structure; providing a mold for encapsulating the electronicassembly and for forming at least a portion of an exterior shape of theencapsulated electronic device with cured encapsulant; providing amoveable insert in a portion of the mold, the moveable insert having aninsert surface for engaging and applying pressure to the firststructure; closing the mold around at least a portion of the electronicassembly; positioning the first structure relative to the secondstructure, the positioning comprising clamping the second structurerelative to the mold and biasing the first structure in position againsta predetermined feature of the mold; and filling the mold with anencapsulating material, wherein the second structure comprises a printedcircuit board (“PCB”) and the first structure comprises a core structurefor directing a magnetic field along a flux path, the core structurehaving a first core surface and wherein the predetermined feature is afirst inner surface of the mold.
 28. The method of claim 27 wherein: thefirst inner surface of the mold is contoured to produce a resulting flatexterior top surface for the electronic device after encapsulation, thefirst core surface is essentially flat, and the positioning comprisesmoving the first core surface against the first inner surface forming aseal to leave the first core surface exposed within and coplanar withthe resulting flat exterior top surface.
 29. The method of claim 28wherein the mold insert is provided in a bottom portion of the mold,adapted to push the core structure upward against the first innersurface, the insert surface is adapted to form a seal against a secondsurface of the core structure to leave the second surface of the corestructure exposed within a resulting exterior bottom surface of thedevice.
 30. The method of claim 28 wherein the mold insert protrudesfrom a bottom interior surface of the mold to produce a recess in theresulting exterior bottom surface of the electronic device afterencapsulation; and the core structure is exposed within the recess. 31.The method of claim 30 further comprising adapting the core structure tomove over a range relative to the PCB; and providing a minimum clearancebetween the core structure and the PCB over the range.
 32. The method ofclaim 31 wherein the encapsulated electronic device comprises a powerconverter.